Modern mobile communications systems use code-division multiple access (CDMA) techniques to allow a plurality of terminals to transmit signals over the same physical media and share its capacity. To make faster broadband wireless communication possible, the 3rd Generation Partnership Project (3GPP) and other standardization committees have eagerly discussed next-generation mobile communications systems, in addition to improvement of the current systems.
Some mobile communications systems allow a mobile station to communicate concurrently with multiple radio base stations. Particularly, for enhanced quality of transmission in the uplink direction (i.e., from mobile station to radio base station), some of such communications systems permit a mobile station to send identical data to two or more radio base stations in a parallel fashion. Those radio base stations are managed by an intermediate network device called a radio network controller or base station controller. A plurality of data units with the same content flow into this intermediate network device, from the mobile station via different transmission paths (called “branches”) between them. The intermediate network device selects and combines appropriate data units according to the transmission quality of each branch, thus sending the selected data to an upper network system (called “core network”). For details, see the following technical specifications of the 3rd Generation Partnership Project: (1) “FDD Enhanced Uplink; Overall description; Stage 2”, 3GPP TS25.309, 2006-03, V6.6.0., (2) “UTRAN Iub/Iur interface user plane protocol for DCH data streams”, 3GPP TS25.427, 2007-09, V7.5.0.
The above intermediate network device discards incoming data that has not been selected. In the case where sufficient quality of communication can be achieved by using some of the available branches (rather than all of them), the intermediate network device discards every data received from other branches. This could lead to a great disadvantage to the efficiency of data transport. Suppose, for example, that the mobile station is using three branches with a bandwidth of 5.8 Mbps per branch, or 17.4 Mbps in total. In the case where one of these three branches provides a sufficiently good quality, the other branches of 11.6 Mbps are eventually wasted. In addition, the intermediate network device has to deal with an increased processing load, which could result in an extra transmission delay and packet loss.
Several researchers have proposed a method to reduce the amount of data flowing into an intermediate network device. See, for example, Published Japanese Translation No. 2007-502559 of PCT Patent Application No. PCT/EP2004/006560. According to this proposed method, each radio base station monitors quality of a radio link used to communicate with a mobile station, thus identifying which radio base stations, if any, are experiencing poor radio conditions. The method then restricts those radio base stations from forwarding received data to the intermediate network device.
However, quality of a radio link section is not the only factor that determines quality of data flowing into an intermediate network device from each branch. Suppose, for example, that a radio base station experiences congestion in its connection to the intermediate network device. Under this situation, quality degradation would be observed in the data received from the radio base station even if the corresponding radio link is in good condition. The conventional method proposed in the aforementioned PCT patent application is unable to adequately determine which radio base stations should stop forwarding data from the mobile station.